The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.11 standard is a family of standards for wireless local area networks (WLAN) in the unlicensed 2.4 and 5 Gigahertz (GHz) bands. The current IEEE 802.11b standard (also known as “Wi-Fi”) defines various data rates in the 2.4 GHz band, including data rates of 1, 2, 5.5 and 11 Megabits per second (Mbps). The 802.11b standard defines single-carrier packets using a serial modulation technique and direct sequence spread spectrum (DSSS) with a chip rate of 11 Megahertz (MHz). The IEEE 802.11a standard defines multi-carrier packets with data rates of 6, 12, 18, 24, 36 and 54 Mbps in the 5 GHz band using an orthogonal frequency division multiplexing (OFDM) encoding method. It is noted that systems implemented strictly according to either the 802.11a standard or the 802.11b standard are incompatible and not designed to work together.
A new IEEE standard is being proposed, referred to as 802.11g (the “802.11g proposal”), which is a high data rate extension of the 802.11b standard at 2.4 GHz. It is noted that, at the time of this disclosure, 802.11g is only a draft standard and is not yet a completely approved standard. It is desired that 802.11 g devices be backwards compatible with 802.11b devices and operate in the 2.4 GHz band. In accordance with a current draft of the 802.11g standard, in fact, 802.11g devices should be configured to fully support communications according to 802.11b and be able to communicate at any of the standard 802.11b rates. It is also desired, however, that the 802.11g devices be able to communicate at higher data rates, such as the same data rates supported by the 802.11a standard. The higher data rates are achieved by borrowing encoding and modulation techniques of 802.11a and applying them in the 2.4 GHz band. The current 802.11g standard includes several higher data rate modes, including a mandatory mode and two optional modes. The mandatory mode employs 802.11a-type packets using OFDM in the 2.4 GHz band. The first optional mode employs mixed-mode packets with legacy CCK preambles followed by OFDM payloads. The second optional mode employs an extension of the Packet Binary Convolutional Coding (PBCC) optional mode of the 802.11b standard using 8PSK (8 Phase Shift Keying) to achieve a higher raw data rate of 22 Mbps.
It is appreciated that a significant technical challenge is presented for 802.11g devices in that they should be configured to detect and acquire different types of packets received in the same radio frequency (RF) band, possibly within the same WLAN. The general 802.11 packet format includes a packet preamble, which is followed by a packet header, which is followed by a packet payload, which may be followed by a packet tail. The packet payload includes packet user information or user data. The packet tail includes packet end signaling, such as pad bits, flush bits, SIFs extension, etc. The 802.11g receivers, however, do not have prior knowledge of which packet-type is being received. The encoding and modulation varies with packet-type and even within a packet depending upon the packet-type. The preamble and header portions vary depending upon packet-type. The signal to noise ratio (SNR) of transmitted packets may vary significantly. To complicate matters, the target 2.4 GHz band may be occupied by non-802.11 sources of interference, such as, for example, microwave ovens, Bluetooth devices, and other interference sources. Each receiver is tasked to detect both weak and strong signals, to distinguish and to decipher the packet-type during the time period of the packet preamble and/or header and before the beginning of the user information in the packet payload of each packet.
Although the present invention is described with reference to the 802.11 standards, the present invention is not limited to 802.11 and applies to any system employing different packet types occupying the same RF band, including, for example, wireless systems using both single-carrier and multi-carrier modulation schemes. It is desired to provide a wireless receiver that detects a packet in a received signal and that determines the packet type before the start of information contained within the packet, such as within the header. It is also desired that the receiver be able to detect signals of varying strength (from weak to strong) and to avoid false detections given other interference sources. It is further desired to use a minimum amount of signal processing circuits and to minimize or otherwise eliminate additional power consumption.